1. Field of the Invention
The present invention relates to a rear projection screen that is used as a component part of a rear projection system such as a projection television. More particularly, the present invention relates to a rear projection screen capable of displaying a sharper image by reducing deterioration which the displayed image undergoes, such as coarsening, flickering and glaring, and to a diffusing sheet for use in such a rear projection screen.
2. Background Art
A conventionally-known rear projection screen of the above-described type comprises: a diffusing part containing light-diffusing particles, provided for increasing the viewing angle; and a Fresnel lens part provided for decreasing the difference in screen brightness between the center and the edges. Another widely-known rear projection screen comprises, in addition to the above-described diffusing part and Fresnel lens part, a lenticular lens part that is provided to more finely control the viewing angle.
A projector for use in a rear projection system in which such a rear projection screen as is described above is incorporated has been improved in recent years and can now project image light with higher brightness and definition. Thanks to this improvement, the color purity of R, G and B light has been increased, and the emission wave range for each color is now getting narrower. As a result, the coherence (chances of causing interference) of image light tends to increase.
The Fresnel lens part contained in the above-described rear projection screen acts like a slit to make the brightness of light emerging from this lens part vary in coincidence with the concentric circular lenses on this lens part. Namely, in such a rear projection screen, when image light projected from a projector passes through the Fresnel lens part, the light rays emerging from the concentric circular lenses on the Fresnel lens part are to interfere with each other. Now, if the rays of such image light with high coherence as is described above pass through the Fresnel lens part and interfere with each other, the difference in intensity between the rays becomes greater, and the whole image displayed on the screen appears coarse, flickering or glaring.
Further, the above-described problem surfaces more clearly as the resolution of the projector becomes higher. There is now a heavy demand for measures to solve this problem.
In order to solve the image-glaring problem described above, there has been proposed a technique that a thin Fresnel lens sheet is supported by interposing it between a rigid light-diffusing sheet and a lenticular lens sheet (see Japanese Patent Laid-Open Publication No. 133508/1999).
This conventional technique, however, considerably restricts the construction of a rear projection screen, so that it has been difficult to freely make an optical design for a rear projection screen or a design for a rear-projection-screen-supporting mechanism. More specifically, this technique has the following two problems: (1) a rigid, thick light-diffusing sheet is used in addition to a Fresnel lens sheet and a lenticular lens sheet, and both surfaces of this light-diffusing sheet reflect incident light to cause loss of light, so that 8% or more of image light is lost to decrease the screen brightness; and (2) since a thin Fresnel lens sheet is interposed between the other optical sheets (a light-diffusing sheet and a lenticular lens sheet), this thin lens sheet readily wrinkles as the two optical sheets expand or shrink differently with changes in humidity or temperature; in order to overcome this drawback, it is necessary to specially devise a rear-projection-supporting mechanism.